Stents, grafts and a variety of other endoprostheses are well known and used in interventional procedures, such as for treating aneurysms, for lining or repairing vessel walls, for filtering or controlling fluid flow, and for expanding or scaffolding occluded or collapsed vessels. Such endoprostheses can be delivered and used in virtually any accessible body lumen of a human or animal, and can be deployed by any of a variety of recognized means. One recognized indication of endoprostheses, such as stents, is for the treatment of atherosclerotic stenosis in blood vessels. For example, after a patient undergoes a percutaneous transluminal coronary angioplasty or similar interventional procedure, an endoprosthesis, such as a stent, is often deployed at the treatment site to improve the results of the medical procedure and to reduce the likelihood of restenosis. The endoprosthesis is configured to scaffold or support the treated blood vessel; if desired, the endoprosthesis can also be loaded with a beneficial drug so as to act as a drug delivery platform to reduce restenosis or the like.
The endoprosthesis is typically delivered by a catheter delivery system to a desired location or deployment site inside a body lumen of a vessel or other tubular organ. To facilitate such delivery, the endoprosthesis must capable of having a particularly small cross profile to access deployment sites within small diameter vessels. Additionally, the intended deployment site is often difficult to access by a physician and involves traversing the delivery system through the tortuous pathway of the anatomy. It therefore is desirable to provide the endoprosthesis with a sufficient degree of longitudinal flexibility during delivery to allow advancement through the anatomy to the deployed site.
Once deployed, the endoprosthesis should be capable of satisfying a variety of performance characteristics. The endoprosthesis should have sufficient rigidity or outer bias when deployed to perform its intended function, such opening a lumen or supporting a vessel wall. Similarly, the endoprosthesis should have suitable flexibility along its length when deployed so as not to kink or straighten when deployed in a curved vessel. It also may be desirable to vary the rigidity or flexibility of the endoprosthesis along its length, depending upon the intended use. Additionally, it may be desirable for the endoprosthesis to provide substantially uniform or otherwise controlled coverage, e.g., as determined by the ratio of the outer surface of the endoprosthesis to the total surface of the vessel wall along a given length. For example, increased coverage may be desired for increased scaffolding, whereas decreased coverage may be desired for side access to branch vessels. Control of the cross profile and length of the endoprosthesis upon deployment also is desirable, at least for certain indications.
Particularly, tradeoffs are traditionally required between device performance during the interventional procedure, in which an endoprosthesis is placed in a vessel, and long term device performance. Excellent placement performance (deliverability, ease of access, stent retention, etc.) favors a stent design that is highly flexible and that has a low profile. Long-term device performance (e.g., coverage, scaffolding, low restenosis) often requires a stent with significant rigidity or outer bias to support the vessel. High scaffolding stents employ a relatively large amount of metal, and this metal can restrict how tightly the stent can be crimped, thus limiting its profile, retention on an expansion balloon, and deliverability performance. The following formula illustrates the relationship between inner diameter (ID) and strut width and number (i.e., scaffolding) in a particular cross-section of a traditional crimped stent, where n is the number of struts and w is the width of each strut:
  ID  =            circumference      π        =          nw      π      
The deliverability of a stent device can be also limited by the amount of material at the distal end of the delivery catheter. The force exerted by stent geometry generally corresponds to the amount of strain in the material, which increases as the stent geometry is deformed from the set state. A traditional stent that is set, such as by heat, in the expanded state exerts the greatest force when it is the most crimped, and a typical stent that is heat set in the crimped state exerts the greatest force when it is the most expanded. The material of typical stents either keeps the stent crimped on the delivery system during delivery or expands the stent at the site of treatment. Traditional stents can only accomplish one of these two tasks and require significant additional material on the delivery system to accomplish the other. Balloon expandable stents, typically made of stainless steel, have mechanical properties allowing them to be easily and securely crimped on a delivery catheter. However, they require a relatively bulky balloon to expand them into the vessel wall. On the other hand, self-expanding stents made of NiTi alloy or other super-elastic materials readily deploy themselves at the site of treatment but use a relatively bulky sheath to keep them constrained on the delivery system during delivery. Both traditional catheter balloons and stent sheaths tend to add significant profile and stiffness to the distal end of the implantation device.
Significant research effort has been devoted to the task of developing higher performance balloons (lower profile, more flexible) to minimize their impact on the delivery of the system. Similarly much work has focused on minimizing the impact of a constraining sheath for self-expanding stents. The use of such low profile, highly flexible delivery system could be furthered by the development of a stent or similar endoprosthesis that requires less force to maintain mounted on and deployed from the delivery system.
Numerous designs and constructions of various endoprosthesis embodiments have been developed to address one or more of the performance characteristics summarized above. For example, a variety of stent designs are disclosed in the following patents: U.S. Pat. No. 4,580,568 to Gianturco; U.S. Pat. No. 5,102,417 to Palmaz; U.S. Pat. No. 5,104,404 to Wolff; U.S. Pat. No. 5,133,732 to Wiktor; U.S. Pat. No. 5,292,331 to Boneau; U.S. Pat. No. 5,514,154 to Lau et al.; U.S. Pat. No. 5,569,295 to Lam; U.S. Pat. No. 5,707,386 to Schnepp-Pesch et al.; U.S. Pat. No. 5,733,303 to Israel et al.; U.S. Pat. No. 5,755,771 to Penn et al.; U.S. Pat. No. 5,776,161 to Globerman; U.S. Pat. No. 5,895,406 to Gray et al.; U.S. Pat. No. 6,033,434 to Borghi; U.S. Pat. No. 6,099,561 to Alt; U.S. Pat. No. 6,106,548 to Roubin et al.; U.S. Pat. No. 6,113,627 to Jang; U.S. Pat. No. 6,132,460 to Thompson; and U.S. Pat. No. 6,331,189 to Wolinsky; each of which is incorporated herein by reference.
Although the various designs for endoprostheses that have been developed to date may address one or more of the desired performance characteristics, there remains need for a more versatile design for an endoprosthesis that allows improvement of one or more performance characteristics without sacrificing the remaining characteristics.